The present invention relates to an aluminum-coated structural member and a production method thereof. More specifically, the present invention relates to an aluminum-coated structure member, such as a structural member used for motor vehicles, produced by applying heat treatment and forming operation to aluminum-coated steel sheet, and coated with an alloy layer of an Alxe2x80x94Sixe2x80x94Fe alloy for improvement in formability, weldability and corrosion resistance.
A Published Japanese Patent Application Publication No. 2000-204463 shows a hot-dip aluminum-coated steel sheet which can be used for motor vehicles as fuel tank, muffler, heat shield plate, etc., from the viewpoint of excellent corrosion resistance due to oxide film, and less burden on the environment due to non-inclusion of heavy metal such as lead.
A hard Alxe2x80x94Fe alloy layer contained in a hot-dip aluminum-coated steel sheet at an interface with a steel substrate is more or less problematical in the formability. Moreover, the alloy layer tends to suffer cracks in a forming operation, and hence sometimes entails another treatment to ensure sufficient corrosion resistance. Weldability is another problem because an aluminum coating layer is low in melting point, and an aluminum single surface layer is insufficient in thermal conductivity. Accordingly, the hot-dip aluminum-coated steel sheet is limited in application.
It is an object of the present invention to provide aluminum-coated structural member and production method to the advantage of corrosion resistance, formability and weldability and wider application in motor vehicles.
According to one aspect of the present invention, an aluminum-coated structural member comprises: a steel substrate layer; and an Alxe2x80x94Sixe2x80x94Fe alloy layer formed on a surface of the steel substrate layer, the alloy layer including a softer region having a hardness smaller than or equal to a hardness of the steel substrate layer, extending from the surface of the steel substrate layer toward a surface of the alloy layer, and having a thickness greater than or equal to 50% of a thickness of the alloy layer, the Alxe2x80x94Sixe2x80x94Fe alloy layer having an oxide weight smaller than or equal to 500 mg/dm2.
According to another aspect of the invention, a production method of an aluminum-coated structural member comprises: a first method element of heating a hot-dip aluminum-coated steel sheet at a heating rate in a range of 1xcx9c10xc2x0 C./sec; a second method element of holding the hot-dip aluminum steel sheet at a raised temperature in a temperature range of 900xcx9c950xc2x0 C. for a duration in a range of 2xcx9c8 minutes; a third method element of cooling the hot-dip aluminum-coated steel sheet to a temperature in a temperature range of 700xcx9c800xc2x0 C. at a cooling rate in a range of 5xcx9c15xc2x0 C./sec; a fourth method element of forming the hot-dip aluminum-coated steel sheet into a predetermined shape in the temperature range of 700xcx9c800xc2x0 C.; and a fifth method element of cooling the hot-dip aluminum-coated steel sheet in the predetermined shape rapidly from the temperature range of 700xcx9c800xc2x0 C. to a lower temperature lower than or equal to 300xc2x0 C. at a cooling rate in a range of 20xcx9c100xc2x0 C./sec.